How long do most running backs keep their "beast" status? One year? Four years?

I'm trying to predict how long Portis can be Mr. Dependable before he is a supplemental back then an over-the-hill back. Thankfully, many things help determine this.

The first is running style. Portis is a downhill runner that can run people over if he wants to, but he needs good blocking to take out big defenders. If that happens, then he is most effective.

The thing is, Portis embraces contact and is rather glad to lower his shoulder, which is great for the play or the game. But, in long term, all those hits could end his career a year short. Ex-Seahawks running back Shaun Alexander is a prime example of this. While he had his MVP season, he is now backup to Portis' backup, Ladell Betts. Was taking all of those hits worth it?

The second factor is the system. The Redskins have changed their system more than 13-year olds change their kissing partners. Every time Portis settles into a system, it is swept out from under him.

Walter Payton did so well because he had stability in his career. Emmit Smith was the same until he became a player who you had to force to retire. Those two went against the odds and lasted longer than many should have.

Thirdly, college truly is a factor. Portis went to the University of Miami and was fantastic. He was a Reggie Bush-like runner who was great at what he did. It was doubtful he would have a successful NFL career because of his size, but the Broncos took a chance on him in the second round.

Portis responded with an amazing rookie season and a sophomore season just as comparable, with 14 touchdowns and 15 touchdowns, respectively.

While he is a great player now, I see him having two more seasons of greatness, one of effectiveness, and three where his fans yell at him to hang up his cleats once and for all.